Add process table and exit syscall

[vinc]

Improve process structure and use a table to save them.

This will allow use to save the registers of a process before spawning a new one so we can switch back to it later.

[vinc]

Debug message in the EXIT syscall:

[vinc]

Two issues:

• We can't boot with 32 file handles per process, so we have to carefully manage 16 file handles
• The boot is too slow with the release flag so the tests fail

[vinc]

Finally got exit working so we can run a program with spawn and go back to the shell, but somehow it only work with the debug output activated.

[vinc]

We can now run a program multiple times and unmap the memory after use, well except sometime the unmapping doesn't work. And sometime the compiled code trigger a reset.

[vinc]

The new exit syscall work well to return to the kernel shell for flat binaries, but some issues remains for elf binaries that will be addressed next.